Recently, next-generation mobile communication systems have been actively studied. As a method for enhancing the frequency utilization efficiency of a system, a single-frequency reuse cellular system has been proposed in which respective cells use the same frequency band so that the cells can use the entire band allocated to the system.
Orthogonal frequency division multiple access (OFDMA) is the most prominent candidate for downlink (communication from a base station device to a mobile station). OFDMA is a communication system in which information data is modulated by use of different modulation schemes, such as 64-ary quadrature amplitude modulation (64QAM) and binary phase shift keying (BPSK), according to reception conditions to generate an OFDM signal, and radio resources defined by time and frequency axes are flexibly allocated to a plurality of mobile user devices.
Since an OFDM signal is used in this case, a peak to average power ratio (PAPR) may greatly increase and peak power may increase. The high peak power is not a large problem for downlink communication having a relatively high transmission power amplification function, but there is a fatal problem in that a signal to be transmitted is distorted since peak power exceeds a linear region of an amplifier (AMP) upon amplification for uplink communication (from the mobile station to the base station device) having a low transmission power amplification function.
Thus, a single-carrier-based communication scheme with a low PAPR is suitable for the uplink (communication from the mobile station to the base station device).
However, the use of the single-carrier scheme has a problem in that flexible resource allocation using time and frequency axes may not be performed such as in the case of OFDM. As a communication scheme for solving the problem, single carrier-adaptive spectrum allocation (SC-ASA) (also referred to as discrete Fourier transform-spread OFDM (DFT-S-OFDM)) has been proposed. (see, for example, Non-Patent Document 1).
Since such a communication scheme uses the same technique as the single-carrier communication scheme, a PAPR becomes low. It is possible to process data without inter-block interference by inserting a cyclic prefix (CP) as in an OFDM signal (hereinafter, an interval at which a CP is inserted, that is, a data processing unit in which a DFT is performed, is called a “DFT-S-OFDM” symbol). Since frequency waveforms are first produced by a DFT, there is a merit in that resource control may be easily performed in a subcarrier unit.
FIGS. 17A and 17B are diagrams illustrating the concept of the SC-ASA scheme. FIG. 17A shows a transmission spectrum. A spectrum of original transmission data converted by time-to-frequency conversion into a frequency signal is arranged in a continuous frequency as shown in a graph shown on the left of FIG. 17A. In the SC-ASA scheme, a spectrum is transmitted by remapping subcarriers as in a graph shown on the right of FIG. 17A after selecting the subcarriers of which a reception situation (reception quality) is good at a receiver. A graph shown on the left of FIG. 17B shows a reception spectrum, and a frequency signal received as in a graph shown on the right of FIG. 17B can be recovered to the original by performing demapping to the same sequence as that of the original transmission data. That is, transmission characteristics are improved since a frequency of a good reception situation can be adaptively selected and transmitted.
FIGS. 18A and 18B are schematic block diagrams showing the configurations of a transmission station device and a reception station device which transmit information by applying the SC-ASA communication scheme. In the transmission station device of FIG. 18A, a transmission bit sequence is coded by an encoding unit 1000, and coded transmission bits are rearranged by an interleaving unit 1001 and modulated by a modulation unit 1002. After a serial/parallel (S/P) conversion unit 1003 converts a modulation signal modulated by the modulation unit 1002 into parallel signals, a DFT unit 1004 converts the parallel signals into signals on a frequency axis. Thereafter, a spectrum mapping unit 1005 maps the signals on the frequency axis to subcarriers. At this time, in a process of mapping to the subcarriers, subcarriers of a frequency of which a reception situation, for example, a signal to noise ratio (SNR) or a signal to noise interference ratio (SNIR), is good are allocated to the frequency, and also 0 is input to unallocated subcarriers.
Next, an inverse IDFT (inverse discrete Fourier transform) unit 1006 converts the mapped transmission signals on the frequency axis into signals on a time axis, and a parallel/serial (P/S) conversion unit 1007 converts the signals on the time axis into a serial signal. Simultaneously, a pilot signal generation unit 1008 generates a pilot signal for estimating a frequency characteristic of a propagation channel, and a pilot multiplexing unit 1009 multiplexes the pilot signal with the serial data signal by conversion of the P/S conversion unit 1007. A CP insertion unit 1010 inserts a CP into a multiplexed signal. The signal into which the CP is inserted is converted by a D/A conversion unit 1011 into an analog signal, up-converted by a radio unit 1012 into a radio frequency, and transmitted from a transmission antenna 1013 in each transmission station.
In the reception station device of FIG. 18B, a received signal is received by a reception antenna 1100. A radio unit 1101 down-converts the received signal into a baseband signal. An A/D conversion unit 102 converts the down-converted received signal into a digital signal. Next, a CP removal unit 1103 removes a CP from the digital signal, and a pilot separation unit 1104 separates a pilot signal for estimating a propagation channel characteristic and a data signal. A propagation channel estimation and noise variance estimation unit 1105 calculates a frequency characteristic of a propagation channel and a variance of noise from the separated pilot signal.
A propagation channel characteristic demapping unit 1106 extracts only a frequency characteristic actually used for transmission from the estimated frequency characteristic of the propagation channel, and a discrete frequency selection unit 1107 selects a discrete frequency of which a reception situation is good. At this time, a spectrum allocation process calculates reception situations of discrete frequencies and selects discrete frequencies to be used in order from a frequency having a high gain. A spectrum allocation information generation unit 1108 generates an allocation information signal of the next transmission opportunity from determined spectrum allocation, and feeds back the allocation information signal to a transmission device.
On the other hand, an S/P conversion unit 1109 converts the separated data signal from which the CP is removed into parallel digital signals. A DFT unit 1110 converts the parallel digital signals into frequency-axis signals, and a spectrum demapping unit 1111 forms the same spectrum sequence as that of the original transmission signal by returning subcarriers of the frequency-axis signals to the original arrangement. Thereafter, an equalization unit 1112 performs an equalization process of compensating for distortion by a propagation channel, and an IDFT unit 1113 converts the frequency-axis signals into time-axis signals. A P/S conversion unit 1114 converts the time-axis signals into a serial signal, and a demodulation unit 1115 performs demodulation into reliabilities (likelihoods) of code bit units from a modulated signal. Finally, a deinterleaving unit 116 returns a coded transmission bit sequence to the original from the likelihood of each code bit, and a decoding unit 1117 obtains decoded data of a signal transmitted from a transmission station.
Non-Patent Document 1: Mashima and Sampei, “A Study on Broadband Single Carrier Transmission Technique using Dynamic Spectrum Control,” RCS 2006-233, January 2007.